1. Field of the Invention
This invention relates to apparatus for recovering charged ions from solution, and, more particularly, to a novel spacer for use in assembling multicell electrodialysis stacks.
2. Description of the Prior Art
Electrodialysis is a well known electrochemical process whereby a solution to be treated is passed between an anode and cathode. Appropriate ion-permeable membranes are placed between the solution to be treated and the respective electrodes so that the anions tend to be drawn toward the anode and cations toward the cathode. Thus, a feed stream being treated is diluted with respect to these ions. On each side of the compartments through which the feed stream is caused to flow, that is on the other side of each ionic membrane from the feed stream, is a "concentrate" or "product" compartment wherein the ions are concentrated. Specially designated spacers separate the membranes in an attempt to form leak-tight flow directing compartments between adjacent membranes, and the entire assembly is held in compression by a pair of end plates and tie-rods or other clamping devices. The compartments or cells formed by the spacers are typically 1-1.5 mm thick and direct the solution fed to the cell over the surfaces of the membranes either in torturous path or sheet flow pattern. The flow path usually contains a plastic screen or supporting baffles to insure separation of the membranes and induce turbulent flow. A stainless steel sheet is commonly used for the cathode and platinized titanium for the anode.
An example of such an electrodialysis stack is shown in U.S. Pat. No. 3,878,086, issued Apr. 15, 1975 entitled "Electrodialysis Stack and Spacer for use therein," incorporated herein by reference. That patent describes an electrodialysis stack and the method of constructing the same, wherein a multiply membrane spacer is used, the spacer consisting of relatively thin exterior plies formed of a more rigid material than is the interior ply. These exterior plies are held together by a water resistant pressure adhesive. The resulting spacer is then preferably adhesively bonded to cationic and anionic membranes on each side thereof, and thereby form a part of an essentially solid laminar dialysis stack.
These spacers are then placed in a stack alternating between feed spacers and product spacers to form the entire stack.
The ion-permeable membranes used in such a stack are made of a material which has a relatively low tear strength. Accordingly, attempts to remove the membrane from the spacer result in tearing within the membrane material itself, rather than at the adhesive bond as described in the U.S. Pat. No. 3,878,086 mentioned above at column 2, line 53 through column 3, line 38. As discussed therein, beginning at column 3, line 16, such a stack can be put together without an adhesive bond between the spacer and the membrane when the stack is to be used for test work, to be broken down for inspection, or to be broken down to change the flow characteristics by changing to different turbulence promoters. However, some leaking will almost invariably occur since the materials which make up the spacer are not of sufficient compressibility to provide a relatively tight seal. As stated at column 3, line 26 through line 38, unique stacks can be formed of modules consisting of membranes sealed to spacers as shown at column 5, lines 12 through 25, the preferred method was to use adhesive on both sides of the membranes so that the stack formed a solid block. In that way, leakage to the stack or intrastream "short-circuiting" was asserted to be virtually eliminated.
Although the attainment of a leak-proof stack is the ultimate goal, providing such by using a stack wherein the spacers are adhesively bonded presents a number of problems when servicing is required. Depending upon the type of membrane used and the solution to be treated, the stacks must be disassembled periodically for maintenance and replacement of the membranes and other components. In the situation in which all the membranes and spacers are bonded as a single unit, many of the membranes which might be reused become torn and must be replaced. This is particularly costly when a membrane fails relatively early in the use cycle of the stack. Although it has been suggested to assemble such a stack without using an adhesive to hold membranes in place, such stacks suffer from leakage to the outside with the concomitant waste and loss of material.